What happened
On 8 March 1999, a Shorts 360, registration EI-CPR, was performing a public transport flight departing from Ronaldsway Airport, Isle of Man. During the taxi from Stand No 2 towards Runway 26, the aircraft failed to respond to a required right turn onto the Northern Taxiway. Despite the crew having successfully completed several 90-degree turns earlier in the taxi, the aircraft continued straight, prompting the commander to apply the wheel brakes.
When the wheel brakes failed to decelerate the aircraft, the first officer also applied the brakes with no effect. The commander attempted to use reverse propeller pitch to slow the plane, but the effectiveness of this maneuver was restricted because the flying control gust locks were still engaged, which prevented the power levers from being moved sufficiently. The crew eventually feathered the propellers, which allowed some steering and braking capability to return. The aircraft ultimately came to a stop at the edge of the grass area.
There were no fatalities and no injuries to the 3 crew members or 25 passengers, and the aircraft sustained no damage.
The investigation
Following the incident, engineers inspected the aircraft and discovered that the hydraulic fluid reservoir lacked gas pressure. This reservoir uses a piston to separate nitrogen from the hydraulic fluid, creating the pressure necessary to supply the engine-driven pumps and prevent cavitation. An engineer had previously bypassed the pressure gauge line due to a suspected leak, and a deferred defect had been logged to repair the system during a scheduled 'C' check.
During the subsequent maintenance at Exeter Airport, a crack was identified in the flared end of a pipe within a union, located in a concealed area behind the cabin trim. While repairing this leak, technicians noted that the hydraulic fluid appeared milky, indicating potential aeration or moisture contamination. The maintenance team concluded that the lack of reservoir pressure likely caused pump cavitation, which resulted in the loss of braking and steering.
Findings
- The primary cause of the loss of control was pump cavitation resulting from a lack of gas pressure in the hydraulic fluid reservoir.
- A crack in the gas pressure indicating pipework led to the depletion of the reservoir's nitrogen pressure.
- The crew did not utilize the independent emergency brake system, which would have remained functional due to its separate accumulator.
- The maintenance organization noted a separate, unrelated incident involving the parking brake in Dublin, but determined it was not linked to the primary failure.